Monitoring circuit for monitoring a voltage

ABSTRACT

Monitoring circuit for monitoring a voltage, comprising a coil, which is incorporated in a first current branch to be connected to the voltage to be monitored, and an auxiliary relay, the actuating winding of which is incorporated in a second current branch connected in parallel to the first current branch. In the first current branch a first make contact of the auxiliary relay is connected in series with the coil. In the second current branch a first resistor is connected in series with the actuating winding of the auxiliary relay. A series circuit consisting of a second resister and a second make contact of the auxiliary relay is connected in parallel to the actuating winding of the auxiliary relay.

FIELD OF INVENTION

The invention relates to a monitoring circuit for monitoring a voltage,comprising a coil, which is incorporated in a first current branch to beconnected to the voltage to be monitored, and an auxiliary relay, theactuating winding of which is incorporated in a second current branchconnected in parallel to the first current branch.

BACKGROUND OF INVENTION

The requirement imposed on this monitoring circuit is that the coil,which is also referred to as the zero voltage coil, must be in thede-energised state when the voltage to be monitored has a value which isless than or equal to a predetermined percentage of the nominal valuethereof. When the voltage to be monitored increases again the zerovoltage coil must be excited again as soon as the voltage to bemonitored is at a value which is greater than or equal to a secondpredetermined percentage of the nominal value thereof

A monitoring circuit of this type having a hysteresis in the switchingfunction is disclosed in laid-open German Patent Application 1 438 960.

The known monitoring circuit is a relay circuit which consists of afirst relay in series with a resistor. The DC voltage to be monitored isconnected over said series circuit. The known relay circuit alsocomprises a second current branch parallel to the first current branchin which the first relay and the resistor are incorporated. An auxiliaryrelay, in series with a make contact, is incorporated in the secondcurrent branch. Said make contact is a contact of the first relay. Theresistor connected in series with the first relay is bridged by a breakcontact of the auxiliary relay.

The known circuit is so sized that when the DC voltage to be monitoredincreases to 5.1 volt sufficient current flows through the actuatingwinding of the first relay and it is consequently excited. As aconsequence of this the make contact that is connected in series withthe auxiliary relay is closed, so that the actuating winding of theauxiliary relay is also brought into the excited state. The result ofthis is that the bridging break contact for the resistor connected inseries with the first relay is opened. As a result of opening of saidbreak contact the associated resistor in the actuating circuit of thefirst relay is switched in and the current in the actuating winding ofthe first relay is reduced.

SUMMARY OF INVENTION

The aim of the invention is to provide a monitoring circuit of the typementioned in the preamble with which the switching hysteresis isreproducible. Moreover, the monitoring circuit must be easily adjustableand of small volume. Furthermore, the monitoring circuit must be robust.

To this end the invention is characterised in that in the first currentbranch a first make contact of the auxiliary relay is connected inseries with the coil, in that in the second current branch a firstresistor is connected in series with the actuating winding of theauxiliary relay and in that a series circuit of a second resistor and asecond make contact of the auxiliary relay is connected in parallel tothe actuating winding of the auxiliary relay.

The invention has the advantage that only one relay is needed in orderto obtain any desired hysteresis. When the resistance ratings of thefirst and second resistor are low compared with those of the actuatingwinding of the auxiliary relay a more accurate monitoring circuit isobtained which is also not sensitive to changes in temperature.

BRIEF DESCRIPTION OF DRAWING

The invention will be explained in more detail below with reference tothe drawing, in which one embodiment of a monitoring circuit accordingto the invention is shown.

DETAILED DESCRIPTION OF THE INVENTION

The monitoring circuit shown in the drawing serves to monitor thevoltage U. To this end the voltage U to be monitored is connected over afirst current branch in which a coil S (also termed the zero voltagecoil) or electromechanical actuator, a resistor R1 and a make contact a₁connected in series are incorporated. Said make contact a is a contactof an auxiliary relay, the actuating winding A of which, connected inseries with a resistor R2, is incorporated in a second current branchwhich is connected in parallel to the first-mentioned current branch.The auxiliary relay is also provided with a second make contact a₂, bymeans of which the resistor R3 can be connected in parallel with theactuating winding A of the auxiliary relay. The resistor R1 connected inseries with the zero voltage coil S is bridged by a break contact s thatis operated by the coil S. A diode D is also connected over the coil Sfor protection.

The aspect concerned here is a zero voltage coil S, or electromechanicalactuator, which must be in the excited state when the supply voltage is,for example, 70% or more of the nominal value and must be de-energisedwhen the supply voltage is below, for example, 35% of the nominal value.

The invention consists of two parts. One part consists of the zerovoltage coil S and the series resistor R1. When switched out and duringswitching in, said resistor R1 is bridged by the auxiliary contact s, asa result of which the maximum current is able to flow through thewinding of the coil S. When the coil has been activated with the voltageU at an adequate magnitude and with make contact al closed and it is inthe excited state, the break contact s is opened and bridging of theresistor R1 is interrupted. The current through the coil S will then bereduced. It has been found that said first part functions insufficientlyaccurately, in other words the coil S does not become excited orde-energised at the correct voltage levels and the excitation andde-energising points are not reproducible.

The other part of the circuit according to the invention is formed bythe auxiliary circuit which consists of an auxiliary relay with theactuating winding A, a series resistor R2 and a bridging resistor R3which is connected by means of an auxiliary contact a₂ of the auxiliaryrelay in parallel to the coil of the auxiliary relay when the auxiliaryrelay is in the excited state. The requisite switching hysteresis isachieved in this way and it is thus ensured that the auxiliary relaybecomes excited and de-energised at the correct voltage levels. The maincontact al of the auxiliary relay switches the zero voltage coil S oractuator in and out.

The series resistor R2 and the bridging resistor R3 have a resistancerating which preferably is lower than that of the actuating winding A ofthe auxiliary relay. In particular during de-energising, what isachieved in this way is that the de-energising voltage is determinedmainly by the series resistor R2 and the bridging resistor R3. As aresult the circuit is much more accurate and the hysteresis is much moreprecisely defined and the circuit is not greatly affected by temperaturefluctuations.

What is claimed is:
 1. Monitoring circuit for monitoring a voltage,comprising a coil (S), which is incorporated in a first current branchto be connected to the voltage to be monitored, and an auxiliary relay,an actuating winding (A) of which is incorporated in a second currentbranch connected in parrallel to the first current branch, characterisedin that in the first current branch a first make contact (a₁) of theauxiliary relay is connected in series with the coil, in that in thesecond current branch a first resistor (R2) is connected in series withthe actuating winding (A) of the auxiliary relay and in that a seriescircuit of a second resistor (R3) and a second make contact (a₂) of theauxiliary relay is connected in parallel to the actuating winding (A) ofthe auxiliary relay.
 2. Monitoring circuit according to claim 1,characterised in that the resistance rating of the first and secondresistor (R2, R3) is substantially lower than that of the actuatingwinding (A) of the auxiliary relay.